Passenger aircraft designed to fly at altitudes in excess of 10,000 feet are generally required to have sealed cabins supplied with pressurized, temperature conditioned air. Although a substantial portion of the supply air is drawn from the cabin and recirculated, at least some fresh air from outside the cabin must be pressurized and mixed with the recirculated air to make up for leakage and to provide adequate ventilation for the cabin. The apparatus responsible for pressurizing and temperature conditioning fresh supply air is referred to in the aircraft industry as an "air pack."
An air pack uses a compressor for pressurizing the relatively low density air at high altitudes. Typically, the compressor is powered by jet engine bleed air, using an air turbine drive. Since use of engine bleed air for this purpose reduces the operating efficiency of an aircraft, it is desirable to minimize the volume of low density fresh air that the air pack must pressurize or supply to the cabin space. However, it should be apparent that at least some of the cabin air must be vented overboard to avoid the build up of carbon dioxide and other contaminants that result principally from the metabolic activity of the passengers in the aircraft.
Exacerbating the ventilation problem is the contamination produced by cigarette smoke. Although cigarette smoking passengers are generally seated in a limited "smoking area" of the aircraft, there is a growing conceern about the rights of non-smoking airline passengers to breathe air free of cigarette smoke and its associated carcinogenic contaminants. A conventional aircraft ventilation system may be set up to exhaust or vent a fixed percentage of the total return air overboard; however, such systems do not provide means to insure that adequate fresh air is being supplied to properly ventilate the cabin, nor means to avoid venting too much air drawn from the cabin. If too little air is vented overboard and thus too little fresh air is supplied as makeup, the cabin air will eventually become stale and unhealthy. Conversely, if too much air is vented, the system must use excessive engine bleed air to run the air pack compressor, in order to supply pressurized fresh air for maintenance of the cabin pressure within the desired limits.
The prior art ventilation systems used on aircraft have generally only permitted manual control of the amount of air vented from the aircraft. Such systems have not provided means for automatically determining whether more or less cabin air should be vented overboard rather than being recirculated. While maintenance of a comfortable and healthy environment in the aircraft cabin undoubtedly has a higher priority, it is also important to minimize the operating costs of an aircraft. Prior art ventilation systems for aircraft have simply relied upon the subjective evaluation of the flight crew to control the amount of fresh air supplied to the cabin and thus often may fail either to provide proper ventilation or to operate at optimum efficiency.